In recent years, managements of opportunistic infections have become more and more significant more than ever because of an increase in the number of elderly people and immunocompromised patients as a result of advanced chemotherapies or the like. As demonstrated by the fact that opportunistic infections are occurring one after another by different a virulent pathogen, it is shown that the problem of infectious disease will not ends as long as there are underlying diseases that diminish the immune functions of patients. Consequently, new strategies for infectious diseases control, including the problem of drug-resistant pathogen, will be one of the important issues in the soon-to-come aged society.
In the field of antifungal agents, heretofore, for instance, amphotericine B which is based on a polyene skeleton, fluconazole, itraconazole and voriconazole which are based on an azole skeleton, or the like, have been developed for the treatment of deep seated mycoses. Most of pre-existing drugs already available commercially have similar mechanism of action, and currently, the appearance of azole-resistant fungi or the like has been problems.
In recent years, as a 1,3-β-glucan synthetase inhibitor with a novel mechanism, naturally occurring compound-derived cyclic hexapeptides caspofungin and micafungin or the like, have been developed; however, from the fact that these agents only exist in injectable form, they are not yet sufficient practically as antifungal agents.
Since there have been the situations that the pre-existing antifungal agents are insufficient for treatment of the deep seated mycoses, there is a demand and need for development of agents which are based on a novel mechanism and are of high safety. As the related art relevant to antifungal agents based on such a novel mechanism, Patent Documents 1 and 2 describe pyridine derivatives which demonstrates effects against the onset, progress, and persistence of infections by inhibiting the expression of cell wall proteins, inhibiting the cell wall assembly and also adhesion onto cells, and preventing pathogens from showing pathogenicity, with the process which transports GPI (Glycosylphosphatidylinositol)-anchored proteins to the cell wall being inhibited.
In light of this situation, Patent document 3 proposes heterocyclic ring-substituted pyridine derivatives as an antifungal agent that has excellent antifungal action not seen in conventional antifungal agents, and is also superior in terms of its properties, safety, and metabolic stability.
Meanwhile, Non-Patent document 1 discloses compounds represented by the following formula as a prodrug in which a γ-glutamyl group has been introduced.
    Patent document 1: WO 02/04626 Pamphlet    Patent document 2: WO 05/033079 Pamphlet    Patent document 3: WO 07/052,615 Pamphlet    Non-patent document 1: Exp. Opin. Ther. Patents (1995), 5 (9): 873-885